It is well known that polymerizing propylene with higher molecular weight comonomers, i.e. comonomers having more than three carbon atoms per molecule, for example C4-30 α-olefins, can confer beneficial properties on the resulting copolymer as compared with propylene homopolymers, e.g. in terms of crystallization behaviour, melting temperature (Tm) and glass temperature (Tg). However the incorporation rate of the higher molecular weight comonomer is lower than that of propylene and this has limited their use as comonomers, as compared with ethylene, in the preparation of propylene copolymers. Two particular problems that arise from this low incorporation rate are that the polymer product may contain significant residual levels of unreacted comonomer which can result in problems of odour and taste, especially with diene and hexene comonomers, and that the polymerization process is inefficient in its usage of the generally more expensive comonomer since it must be present in the polymerization reactor in a higher molar concentration relative to propylene than that would be the case if the incorporation rate was the same for both monomers.
Thus, for example, while the incorporation rate of higher molecular weight comonomers in propylene copolymers formed using single site catalysts is better than in copolymers formed using Ziegler-Natta catalysts, the comonomer incorporation rate is still considerably less than that of the propylene (see for example WO 01/48034, WO 95/32242 and Forlini et al., Macromol. Chem. Phys. 201: 401-408 (2000)).